The present invention relates to an optical free space interconnect of circuitry. Particularly, the present invention concerns optical interconnection employed in computers.
Expansion slots greatly increase operational characteristics of personal computers (PCs). The expansion slots are connected to various PC circuitry, such as a microprocessor, through a bus and allow the PC to communicate with peripheral devices, such as modems, digital cameras, tape drives and the like. To that end, electrical interface circuitry, referred to as adapters or expansion cards, are inserted in the expansion slots to facilitate communication between the PC circuitry and the peripheral devices. The combination of expansion slots, expansion cards and bus system is commonly referred to as a backplane interconnect system. The bus system associated with the backplane interconnect system connects power, data and control lines to the expansion cards and facilitates communication between the expansion cards and other PC circuitry. The bus system cooperates with a protocol to, among other things, prevent two or more expansions cards from concurrently communicating on a common bus line.
Referring to FIG. 1, an example of a prior art backplane interconnect system 10 includes expansion slots 12 mounted on a motherboard 14. The expansion slots 12 are wired together with one or more busses 16 disposed on the motherboard 14. Each bus 16 normally has multiple lines with terminations 18 at opposing ends of each line. The expansion card 22 has a mating connector 20 that is adapted to be received into the expansion slot 12. Each expansion card 22 may contain numerous circuits and components 24 to perform desired functions. The circuits and components 24 are in electrical communication with conductive traces 26 on the mating connector 20 through bus transceivers 28. Bus transceivers 28 facilitate communication between components 24 of the various expansion cards 22 in backplane interconnect system 10 by driving and detecting signals on the bus lines 16.
As the operational speed of PCs increases, the need to increase the data transfer rate over the backplane interconnect system becomes manifest. Conventionally, increases in data transfer rate have been achieved by either increasing the operational frequency of the individual expansion boards or by increasing the number of lines associated with a bus. Increases in data transfer rates of backplane interconnect systems have been inhibited by crosstalk, noise, degradation in signal integrity and the operational limitations of connectors. One attempt to increase the data transfer rates of a backplane interconnect system has been directed to controlling the impedance associated with the bus lines, as discussed in U.S. Pat. No. 6,081,430 to La Rue. However, it has been recognized that optical backplanes have been successful in increasing the data transfer rates of backplane interconnect systems.
U.S. Pat. No. 6,055,099 to Webb discloses an optical backplane having an array of lasers in optical communication with a lens relay system. The lens relay includes a series of coaxially aligned lenses. The lenses are spaced apart along a planar substrate and form repeated images of an optical array at the input to an interconnect. Output ports are located at different points along the interconnect. Each pair of lenses encloses one of the repeated images and is formed as a single physically integral member. The integral member may take the form of a transparent rod having spherical end surfaces. Each of the spherical end surfaces then provided one of the pair of lenses.
U.S. Pat. No. 5,832,147 to Yeh et al. discloses an optical backplane interconnect system employing holographic optical elements (HOEs). The backplane interconnect system facilitates communication with a plurality of circuit boards (CBs) and a plurality of integrated circuit chips. Each CB has at least an optically transparent substrate (OTS) mate parallel to the CB and extending outside a CB holder. On another OTS mate, two HOEs are utilized to receive and direct, at least part of, a light beam received to a detector on a corresponding CB via free space within the circuit board holder or reflection within the OTS mate. A drawback with the prior art optical backplane interconnect system is that the number of optical channels that may be provided is limited due to the difficulty in achieving discrimination between optical free space signals.
What is needed, therefore, is an optical backplane interconnect system that increases the number of optical channels while avoiding crosstalk in optical signals propagating along the optical channels.
Provided is an optical backplane interconnect system, one embodiment of which features transceiver subsystems employing holographic optical elements (HOEs) that define, and discriminate between, differing optical channels of communication. The HOEs employ a holograph transform to concurrently refract and filter optical energy having unwanted characteristics. To that end, the transceiver subsystem is mounted to an expansion card and includes a source of optical energy and an optical detector. The HOE need not be mounted to the expansion card. In one embodiment, however, the HOE is mounted to the expansion card and in optical communication with either the source of optical energy, the optical detector or both.
The expansion card is in optical communication with an additional expansion card associated with the interconnect system that also includes the transceiver subsystem and HOE discussed above. The source of optical energy is positioned so that the optical detector associated with the additional expansion card senses the optical energy produced by the source, defining a first source/detector pair. A first HOE is disposed between the source and the detector of the first source/detector pair. A second HOE is disposed between a second source/detector pair that includes the optical detector of the expansion card positioned to sense optical energy produced by the optical source of the additional expansion card. The first and second HOEs are formed to limit the optical energy passing therethrough, attenuating all optical energy that impinges thereupon and having unwanted characteristics. In this example, optical energy of the type that is attenuated by the first HOE may propagate through the second HOE, and optical energy of the type attenuated by the second HOE may propagate through the first HOE. In this manner, the first and second HOEs may define differing optical channels by selectively allowing optical energy to pass therethrough. To that end, the first HOE is placed in close proximity with the optical detector of the additional expansion card, and the second HOE is placed in close proximity to the optical detector of the expansion card. Each of the two aforementioned optical detectors would sense only optical energy having desired characteristics. Hence, two discrete optical channels are defined, each of which may be in communication with one or both of the two sources of optical energy.
In another exemplary embodiment, each of the aforementioned optical channels may be uniquely associated with one of the optical detectors and one of the sources of optical energy. To that end, two or more pairs of HOEs are employed. Each HOE of one of the two pairs is associated with a source/detector pair and has holographic transforms that is substantially similar, if not identical, to the holographic transform associated with the remaining HOE of the pair. However, the holographic transform associated with one of the pairs of HOEs differs from the holographic transform associated with the remaining pair of HOEs. In this manner, two optical channels may be defined with crosstalk between the channels being substantially reduced, if not eliminated. With this configuration, the number of optical channels may be increased so that hundreds of optical channels may facilitate communication between two expansion cards, with some of the optical channels being redundant to increase the operational life of the optical backplane interconnect system. These and other embodiments are described more fully below.